1. Field of the Invention
The present invention relates to a polyamide-imide resin insulating paint, and in particular, relates to a low-permittivity polyamide-imide resin insulating paint which can be obtained from a monomer having three or more benzene rings, and an insulation wire using the paint.
2. Description of Related Art
These days, hybrid cars are becoming popular as the result of the energy conservation policies. To improve fuel economy in hybrid cars and motor performance, drive motors are controlled by inverters and rapidly becoming smaller, lighter, highly heat-resistant and are driven under higher voltage. To meet a request for higher motor performance, such as small size, light weight, and high heat resistance, windings currently used for the motor coil require polyamide-imide enameled wires which have excellent heat resistance, mechanical characteristics that can withstand severe coil formation, and mission oil resistance. The mission-oil resistance greatly effects insulation-retaining capacity depending on types and quantities of oil additives. However, if influences of an oil additive are excluded, hydrolyzability due to moisture absorption is directly related to mission-oil resistance.
On the other hand, a polyamide-imide resin insulating paint is a heat-resistant polymeric resin having heat resistance properties, excellent mechanical characteristics, and hydrolyzability resistance properties in which amide groups and imide groups are compounded at a ratio of approximately fifty-fifty. Generally, the polyamide-imide resin insulating paint is created by a decarboxylation reaction of mainly two components, 4,4′-diphenylmethane diisocyanate (MDI) and trimellitic anhydride (TMA), in a polar solvent, such as N-methyl-2-pyrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), and dimethylimidazolidinone (DMI).
The isocyanate method and the acid chloride method are well-known examples of manufacturing methods for the polyamide-imide resin insulating paint. From the viewpoint of manufacturing productivity, the isocyanate method is usually utilized. An example of well-known polyamide-imide resin is one that is generated by a synthetic reaction of mainly two components which are 4,4′-diphenylmethane diisocyanate (MDI) and trimellitic anhydride (TMA) as an acid component. Furthermore, there is a method in which aromatic diamine and aromatic tricarboxylic anhydride react with each other under the acid excess condition at a compounding ratio from 50/100 to 80/100 in order to reform characteristics of polyamide-imide resin, and then polyamide-imide resin is synthesized by using a diisocyanate component (see JP-B-2897186). However, one of disadvantages of the film made of polyamide-imide resin insulating paint is its high permittivity. Due to the resin's structure, the existence of an amide group and an imide group significantly affects the increase in permittivity.
An insulation wire, specifically, an enameled wire used for a motor coil tends to be driven using an inverter to increase efficiency, and accordingly, excessive voltage (inverter surge) is generated causing partial discharge degradation to occur, resulting in insulation breakdown in many cases. Furthermore, motor drive by high-voltage coupled with the superposition of inverter surge increases the risk of the occurrence of partial discharge; therefore, it is becoming difficult to cope with insulation against inverter surge.
As a method of increasing voltage-applied service life by solving the problem of partial discharge, technology of a partial-discharge resistant enameled wire has been disclosed which is manufactured by applying on a conductor a partial-discharge resistant resin paint which is obtained by dispersing organo silica sol in a resin solution (e.g., see JP-B-3496636 and JP-A-2004-204187). There is another method in which an electric field between wires (electric field included in the layers of air present between wires) is eased to prevent partial discharge from occurring, thereby increasing a voltage-applied service life. The above method is classified into two methods: one method in which an electric field is eased by making wire surface conductive or semiconductive, and the other method in which an electric field is eased by decreasing permittivity of the insulation film.
However, the method in which the surface of an insulation wire is made conductive or semiconductive has many problems and is not practical because damage tends to occur during the coil winding process and insulation characteristics are degraded, and an insulation procedure must be conducted on the wire ends. On the other hand, with regard to the method in which permittivity of the insulation film is decreased, since decrease in permittivity depends on the resin structure, it was difficult to acquire both heat resistance and excellent mechanical characteristics simultaneously.
In a method described in JP-B-2897186, if 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) and trimellitic anhydride (TMA) react with each other under the acid excess condition at a compounding ratio of 50/100 for the first stage synthetic reaction, due to the compounding ratio, acid anhydride preferentially reacts with an amino group than carboxylic acid; and furthermore, if a synthetic reaction proceeds, dehydration and imidization will occur, and bis trimellitic imide having carboxylic acid on both ends will be formed.
However, when a compounding ratio of BAPP is more than 50, a reaction between an amino group and TMA's carboxylic acid is hindered; therefore, even if a synthetic reaction is conducted at 200° C. which is the approximate boiling point of NMP, the amino group remains, and results in forming a urea linkage of an amino group and an isocyanate group during the second stage synthetic reaction; consequently, characteristics are aggravated. Furthermore, when a compounding ratio of BAPP is less than 50, acid anhydride remains during the first stage synthetic reaction, water associated with an imidization reaction remains within the system, and acid anhydride turns into carboxylic acid; consequently, reactivity significantly decreases.
The characteristics aggravation problems are derived from improper compounding balance of those functional groups. Accordingly, low-permittivity polyamide-imide which can cope with high-voltage drive is required to obtain excellent enameled wires.